posted by nick .
If a baseball is thrown 30 m/s backwards from a truck moving 50 m/s, how fast will the ball strike the glove of a ground-based catcher?
If the catcher is behind the truck when the ball is thrown, the ball will never get there. I will still have a forward velocity with respect to ground.
If the catcher faces away from the oncoming truck and the ball is thrown before the truck passes, he might have a chance of catching it at 20 m/s relative velocity